(1) Field of the Invention
The present invention relates to a water-soluble acrylic resin capable of forming a coating film showing good water resistance without developing whitening and a resin composition containing the water-soluble acrylic resin for use in a water-based coating composition, and further relates to a water-based coating composition showing good pigment dispersibility and storage stability, and capable of forming a coating film showing good water resistance and adhesion properties and to a coating method using the water-based coating composition as a base coat coating composition.
(2) Description of the Background Art
Although an organic solvent-based coating composition has widely been used for coating an automobile body or the like in the art, recently the use of the organic solvent-based coating composition is being replaced with the use of a water-based coating composition from the standpoints of atmospheric pollution and resources saving. Particularly, in the case of a finish coating method which comprises coating a base coat coating composition followed by coating a clear coating composition, various methods of making the base coat coating composition a water-based one have been proposed (see, for example, Japanese Patent Application Laid-Open No. 193676/87). The water-based coating composition for use in the base coat usually uses resin components such as an acrylic resin emulsion, urethane resin emulsion, water-soluble acrylic resin and the like.
However, in the case where the water-based coating composition containing the above resin component is used as of a cold-drying type, a coating film is formed by fusion-bonding between emulsion particles, resulting in that the resulting coating film shows remaining shapes of emulsion particles. A water resistance test of the above coating film shows that water is localized on an interfacial boundary between emulsion particles to remarkably develop whitening. For the purpose of solving the above problems, studies have been made on a combined use of a water-soluble acrylic resin with the above resin emulsion so as to bridge a gap between emulsion particles with the water-soluble acrylic resin and to improve water resistance with the result that the use of the above water-soluble acrylic resin showed unsatisfactory water resistance.
For the purpose of improving water resistance of the coating film formed by use of the water-soluble acrylic resin, studies have been made on imparting hydrophobic properties to the water-soluble acrylic resin without impairing water solubility, and on use as a comonomer of a hydrophobic monomer such as a monomer having a long chain alkyl group, styrene and the like, resulting in such problems that the use of the monomer having long chain alkyl group causes reduction in a glass transition temperature Tg of the resin, poor high temperature water resistance and a brittle coating film, and that the use of styrene causes remarkable development of whitening during drying of the coating film and development of whitening after drying too, along with such problems that too increase in hydrophobic properties may develop seeding on mixing with a pigment.
On the other hand, a so-called metallic coating by use of a coating composition containing a metal pigment such as aluminum flake or the like, and a so-called solid coating by use of a coating composition containing an organic or inorganic color pigment have been applied to a finish coating of an automobile body and the like. Attempts on the preparation of a water-based coating composition from the above coating compositions produced the following problems.
In the case of the coating composition containing the metal pigment, addition of water causes generation of hydrogen gas and produced problems of safety and storage stability, resulting in making a long period of storage of the above coating composition impossible. For the purpose of solving the above problems, various methods, for example, a method of dispersion-treating a metal pigment such as aluminum flake by use of a phosphate group-containing treating agent, have been disclosed, for example, in U.S. Pat. No. 4,916,176, EP238222, EP319971, U.S. Pat. No. 5,151,125, EP458245, etc. According to the above methods, the metal pigment is coated with a treating agent containing phosphate group, sulfate group or the like to form a water-dispersible pigment paste capable of being added to a water-based coating composition, resulting in making it possible to control the generation of hydrogen gas. Application of the above treating agents to a metal pigment having a small particle size resulted in making it impossible to satisfactorily control the generation of hydrogen gas and in producing problems of water resistance and adhesion properties of the resulting coating film.
In the case of the coating composition containing the color pigment, on preparing a water-based coating composition, use of a dispersion paste prepared by using the conventionally used aqueous acrylic resin as a resin for dispersion produced such problems that an unsatisfactory adsorption of the resin to the color pigment may cause to increase a viscosity of the dispersion paste during storage, resulting in making it difficult to increase a pigment concentration to a high level. Recently, preparation of a dispersion paste having good color conditioning workability and capable of forming a coating film having high clarity and higher transparency has been demanded for use in a base coat coating composition in a finish coating method.